Japanese Patent No. 2618400 describes an optical fiber that includes a cladding layer provided on the periphery of a center core in which the cladding layer has a refractive index groove that has a lower refractive index. It is suggested that optical fibers having such a structure are expected to exhibit various advantageous effects, such as reducing the dispersion slope and lowering the bending loss. However, to achieve such effects, it is desirable that the value of a2/a1 be between 1.5 and 3.5, where a1 is the radius of the core and a2 is the radius of the inner periphery of the refractive index groove.
Conventionally, transmission systems using WDM (wavelength division multiplexing) and optical fibers therefore have actively been developed in order to increase the transmission capacity of trunk lines or long-distance lines. Optical fibers for WDM transmission are required to have certain characteristics, such as reducing nonlinear effects and suppressing the dispersion. In recent years, optical fibers that exhibit a reduced dispersion slope for a system called “metro” in a span of several hundred kilometers and optical fibers that are almost free from loss due to OH have been proposed.
When installment of optical fibers into offices and homes (FTTH; Fiber to the Home) is taken into consideration, characteristics different from those of optical fibers used in transmission are required. That is, when installing optical fibers into buildings or houses, a very small amount of bending of a bending diameter of 30 mm φ or 20 mm φ may be generated. In addition, it is crucial that loss is not increased when a fiber is wound with a small bending diameter to accommodate an excess length. In other words, resistance to a small bending is a crucial characteristic for optical fibers for the FTTH. In addition, good connectivity with optical fibers laid between a base station and buildings or houses (many of which are conventional single-mode fibers for the 1.3 μm band) is also important. Furthermore, low cost is required for such an application.
As optical fibers installed in offices and homes, conventional single-mode fibers for the 1.3 μm band or multimode fibers have been widely used.
However, the allowable smallest bending diameter of such optical fibers is generally about 60 mm φ, and careful attention should be paid to ensure that no bending with a diameter greater than this allowable range is generated when installing the fibers.
Recently, optical fibers having an allowable bending diameter as small as 30 mm φ have been developed by reducing an MFD (mode field diameter) within the range compliant with ITU-T (International Telecommunication Union—Telecom Standardization) G.652, which is an international standard for single-mode fibers (hereinafter, abbreviated as SMFs as appropriate) for the 1.3 μm band.
However, it is desirable for optical fibers that are installed in buildings or houses to have a smaller bending diameter. Although there have been reports about optical fibers with smaller bending diameters, such optical fibers have problems, such as having increased splice loss compared with conventional optical fiber and increased manufacturing costs.
In addition, the Institute of Electronics, Information and Communication Engineers Technical Report OFT 2002-81 reports studies on the possibilities of using photonic crystal fibers in houses or buildings. Photonic crystal fibers are optical fibers having a structure in which holes are provided in the vicinity of the center of optical fibers. Although it is expected that photonic crystal fibers may exhibit characteristics that conventional optical fibers do not have, they are inferior in terms of ease of manufacturing.
In addition, it is desirable that conventional optical fibers used for cables have high bending resistance. For example, for cable layout for connecting cables within a closure, using optical fibers exhibiting resistance to smaller bendings, it is possible to enhance the efficiency of connection and accommodation as well as achieving a reduction in the size of the closure. In addition, the installation task may be performed while communication is taking place through fibers other than the fiber to be installed are active for communication. Even in such a situation, it is possible to perform the task without affecting lines used for communication (live lines) due to unintentional contact.